No configuration changed ? Different CPUs, different memory, different network cards, different connections to the disk devices, difference disk devices ?

How are you measuring the change, and what have you checked so far ?

I hope you have at least got some AWR (or statspack) data for typical activity prior to the migration so that you can compare it with the same activity post-migration. That might give you some clues about where to look.

I have migrated my blue Ferrari to a blue VW Polo. No configuration has changed (same driver and car is still blue). The performance has deteriorated immediately after the migration. Could someone explain why would this happen?

As mentioned, you really need to (actually, should have done, as guess it's a bit late now...) do some proper benchmarking here. But I'll try and have a wild guess at what the problem might be - I'm assuming that someone specified reasonably comparable boxes in terms of performance?

We did a similar migration a few years ago (a few oracle and AIX versions ago too) and had a number of issues, mainly with the correct configuration of virtual memory management on AIX - basically AIX would steal memory off oracle to use for general file caching etc.

I've seen some really weird colours on top end cars locally - that I know for a fact that the manufacturer does not offer on the product brochure when spec'ing the car for purchase... Like pink for a S-class Merc.